Just another comment regarding the performance issue with Moodle 2.4.x: We also had a huge performance issue with our moodle 2.4.6. While the webserver had very low load, the database server was fully using its cpu-ressources. This blocked access to our moodle completely. The problem was a very inneficient sql-query (https://tracker.moodle.org/browse/MDL-35685), which Adrian fixed for us (he backported the 2.5-solution to 2.4). I wonder why this issue has been solved for Moodle 2.5, but not been backported to the Moodle code on 2.4/2.3 ... Not sure whether it's the same issue you discussed here, but might help you.